Jason Richardson's jumper was ridiculed when he first came into the league as well. This is something Rondo can fix. Further, Rondo doesn't need to have three-point range to be effective. If he can hit 16-18 footers off of pull-ups, he will be very effective because he is so good at getting past the initial defender.
Also if he can step into medium range jumpers after dishes from KG and/or Pierce he will help the Cs give the opposition fits. The first thing teams are going to try to do is double KG in the post (leaving Kendrick) and drop Rondo's defender in Pierce's path to the basket when he drives off the inevitable KG pass. If Rondo is able to catch and shoot from Pierce's pass in this scenario, defenses will be hosed.